1. Field of the Invention
The present invention relates to a method for producing optical disks by transferring an asperity pattern formed on a stamper, and optical disks produced by the method.
2. Description of the Related Art
In recent years, technologies of recording still image information and moving image information, as well as character information in media have been developed. Among such technologies, optical disks operated such that laser light is irradiated onto a disk plane to detect the reflected light, and data is read based on the detected reflected light have been widely used for the features of a large recording capacity and long-term storage.
DVDs (Digital Versatile Discs), which are optical disks of a large recording capacity, have been standardized, and the DVDs each having a size 120 mm in diameter, with one side surface having a capacity of 4.7 GB are commercially available. A technique has been proposed to attain further high-density recording, for example, in “Large-capacity optical disk with one side surface having a capacity of 12 Gbyte” written by Yamamoto, O plus E, 20 (No. 2), pp. 183-186 (1998). The article recites that the proposed optical disk has a light-transmissive protective layer of 0.1 mm in thickness, and that the optical disk comprises a thick substrate to secure rigidity of the disk.
There are two proposed ways to increase the recording capacity: one is use of laser light of a shorter wavelength, and the other is use of an object lens having a larger numerical aperture. Use of an object lens having a larger numerical aperture has merits in that the spot of irradiated laser light is decreased, and accordingly, high-density recording is achievable. This technology, however, has demerits in that the focal length may be shortened, and that data reading may be greatly affected by tilting of a light incident plane of the disk relative to the optical axis. In view of these drawbacks, it is required to reduce the thickness of the light-transmissive layer (protective layer) corresponding to the light incident plane.
There is, however, a limit regarding the technology of reducing the thickness of a substrate formed by injection molding. Accordingly, an increase of recording capacity by injection molding is difficult. There has been developed production of optical disks constructed such that a light-transmissive layer capable of transmitting laser light is formed on a substrate, and laser light is irradiated onto a metallic thin film (reflective layer) via the light-transmissive layer. There is defined a recording capacity standard ranging from 23 GB to 27 GB for one side surface regarding Blu-ray disks, which is the next-generation optical disks proposed at present.
Another method for fabricating optical disks is proposed in Japanese Unexamined Patent Publication No. HEI 8-124224, for instance. The publication discloses a method of forming a thermoplastic resin layer on a flat resin base, and heat-pressing a stamper having an asperity pattern against the thermoplastic resin layer. Japanese Unexamined Patent Publication No. HEI 1-138636 discloses a method of forming, on a thin film, a layer made of a UV curing resin, an electron beam curing resin, or a thermosetting resin, and pressing a stamper against the resin layer.
FIGS. 8A through 8D schematically show the process of fabricating optical disks according to the method disclosed in Japanese Unexamined Patent Publication No. HEI 8-124224. Referring to FIG. 8A, a thermoplastic resin layer 102 is formed on a substrate 101. When the surface of the thermoplastic resin layer 102 is heat-pressed against a stamper 103 having an asperity pattern, as shown in FIG. 8B, the asperity pattern of the stamper 103 is transferred to the thermoplastic resin layer 102. Thereafter, as shown in FIG. 8C, a metallic thin film 104 or an equivalent layer, as a reflective layer, is deposited on the thermoplastic resin layer 102 by vacuum deposition, for instance. Lastly, as shown in FIG. 8D, a protective layer 105 is formed on the metallic thin film 104, thus fabricating an optical disk.
As mentioned above, it is required to reduce the thickness of the protective layer 105 in order to increase the recording density of the optical disk. There is another requirement that a thick substrate such as the substrate 101 be bonded in order to secure rigidity as the disk.
The method disclosed in Japanese Unexamined Patent Publication No. HEI 8-124224 comprises heat-pressing the stamper 103 having an asperity pattern against the thermoplastic resin layer 102 formed on the substrate 101, followed by depositing the metallic thin film 104 on the thermoplastic resin layer 102. Forming the metallic thin film 104 after forming the asperity pattern on the thermoplastic resin layer 102 may make it difficult to coincide the asperity patterns on the upper and lower surfaces of the reflective layer 104 with each other. Specifically, whereas the asperity pattern on the lower surface of the metal thin layer 104 facing the thermoplastic resin layer 102 precisely matches with the asperity pattern on the thermoplastic resin layer 102, the asperity pattern on the upper surface of the metal thin layer 104 does not match with the asperity pattern on the thermoplastic resin layer 102 in such a manner that a corner defined by a land and an adjacent groove has a substantially round shape, as shown in FIG. 8C. As a result, when laser light is incident on the metallic thin film 104 from above in the plane of FIG. 8D, unnecessary diffuse reflection is generated due to the round corners of the asperity pattern. In the case where the recording density is low, influence of such diffuse reflection is negligible. However, as the recording density is increased, such diffuse reflection gives adverse effect to signals, which may likely to cause errors in reading.
As a measure to prevent such a drawback, there is proposed an idea of forming an asperity pattern on the protective layer 105 serving as a laser light incident plane, and depositing the metallic thin film 104 on the protective layer 105 having the asperity pattern. However, it is practically impossible, according to the current standard of technology, to manufacture the protective layer 105 having a thickness of 0.1 mm and having an asperity pattern by injection molding as employed in manufacturing the substrate 101. In view of this, there is proposed a method, on the basis of the technology disclosed in Japanese Unexamined Patent Publication No. HEI 8-124224, of coating a thermoplastic resin on a thin sheet to make the thickness of the resin-coated sheet of about 0.1 mm, and heat-pressing a stamper against the resin-coated sheet. This method, however, has a drawback that since the rigidity of the resin-coated sheet is low due to its very small thickness, the degree of heat shrinkage may vary locally owing to non-uniform temperature distribution during the heating. As a result, crinkling may appear over the entirety of the sheet. Likewise, crinkling resulting from shrinkage appears on the optical disks produced by the method as disclosed in Japanese Unexamined Patent Publication No. HEI 1-138636, which comprises the steps of forming a layer of a UV curing resin, a electron beam curing resin, or a thermosetting resin on the sheet, and pressing the stamper against the resin-coated sheet. Furthermore, adhesiveness between the sheet and the resin is weak in the disks produced by the method disclosed in Japanese Unexamined Patent Publication No. HEI 1-138636.